1. Field of the Invention
This invention relates to computers, and more particular, to an electromagnetic safety (EMS) enhancement circuit for a USB (which is acronym for Universal Serial Bus) system, which can, in the event of the USB device being disconnected from the USB interface due to electromagnetic interferences, simulate an unplugging and replugging action for the USB device so as to reconnect the USB device to the USB interface to ensure continuous operation.
2. Description of Related Art
Ever since the IBM Corporation introduced the PC/XT and PC/AT models of personal computers (PC), the IBM-compatible PCs have become the most popular computer systems around the world. A PC is typically designed for use with a variety of peripheral devices to increase the capabilities of the PC. These peripheral devices include, for example, keyboards, printers, mice, modems, external drive systems, external backup systems, scanners, joysticks, to name just a few.
Some peripheral devices can be connected via the standard RS-232 serial port or the parallel port to the PC, and some others may require the installation of special interface cards, such as SCSI (Small Computer System Interface) cards, in the PC. Scanners, in particular, require a dedicated interface card for connection to the PC. In the multimedia age, almost all of the above-mentioned peripheral devices are indispensable to the PC users. Therefore, when a user wishes to connect a great variety of peripheral devices to the PC, the user must first install the associated interface cards, if required, in the expansion slots in the PC, and then connect them via respective cables to the peripheral devices. This practice, however, usually creates a huge twisted body of cables in the back of the PC, which is not only difficult to handle but also makes the office room quite messy.
As a solution to the foregoing problem, the Intel Corporation has introduced a new technology called USB (which is acronym for Universal Serial Bus) which allows the various peripheral device to be connected to the PC through a single, general-purpose port. The peripheral devices that are specifically designed for use with the USB are referred to as USB devices in this specification. The USB also supports Plug and Play (PnP) which allows the PC to automatically configure itself to work with a certain peripheral device right after the peripheral device is plugged into the USB interface. Moreover, the USB support hot plugging which allows a peripheral device to be plugged into the USB interface while the PC is powered on without having to first turn off the PC. When he peripheral device is unplugged, the PC can automatically detect this condition and uninstall all the related drives. These features make the peripheral devices more convenient to use with the PC.
One drawback to USB devices, however, is that they can easily fail the EMS (electromagnetic safety) test due to the reason that the USB system performs data communication with the PC at a transmission rate of 1.5 Mbps or 12 Mbps (megabits per second), which is significantly higher than non-USB devices, thus making the USB devices highly susceptible to electromagnetic interferences. When a certain USB device fails to transmit data correctly for a preset number of times, for example 3, the USB interface will disconnect that USB device. To resume operation, the user needs to manually unplug the USB device from the USB interface and then replugged it again to the USB interface. This practice, however, is unacceptable during the EMS test.
FIG. 1 is a schematic block diagram showing a conventional EMS circuit used in a USB system. As shown, the USB system includes a USB interface 10 and a USB device 12 connected via a cable 14 to the USB interface 10. By specification, the USB interface 10 is provided with a power line V.sub.CC, a data line D+, a complementary data line D-, and a ground line GND. The data line D+ is used to transfer a stream of output data from the USB device 12 to the USB interface 10, while the complementary data line D- is used to transfer the complement of the data being transferred through the data line D+ for data checking purpose by the USB interface 10 to ensure correct data transmission. In addition, the USB interface 10 can use the data lines D+, D- to detect whether the USB device 12 is a high-speed peripheral device or a low-speed one. Further, the USB interface 10 uses the power and ground lines V.sub.CC, GND to supply power to the USB device 12. The cable 14 is constructed on a double-shield structure that allows the data transmission therethrough to be immune to electromagnetic interferences.
The power from the power and ground lines V.sub.CC, GND is transmitted through an inductance-capacitance circuit (LC circuit) consisting of two bead inducters L1, L2 and two capacitors C1, C2. The first inductor L1 has one end connected to the power line V.sub.CC and the other end connected to the USB controller 16; and the second inductor L2 has one end connected to the ground line GND and the other end connected to the USB controller 16. The first capacitor C1 is connected across the input ends of L1, L2; and the second capacitor C2 is connected across the output ends of L1, L2. The first capacitor C1 has a capacitance of about 0.1 .mu.F. When powered on, the USB controller 16 can generate an output data signal with a high-voltage logic state of 3.3 V, which is then transferred via the resistor R1 to the data line D- and also fed back to the USB controller 16. The resistor R1 is, for example, 1.5 k.OMEGA. in resistance. In the case of the USB device 12 being a low-speed peripheral device, the output data signal from the USB controller 16 is transferred to the data line D-; whereas, in the case of a high-speed one, the output data signal is transferred to the data line D+.
One drawback to the conventional EMS provisions in the foregoing USB system, however, is that the double-shield structured cable and the LC circuit are both quite costly to use since this kind of cable is complex in structure and thus difficult to manufacture and the LC circuit requires a number of inductors and capacitors to implement. Moreover, the USB devices can still easily fail the EMS test.